[Android] Support third part media player on Crosswalk

A requirement from an important customer who want to forward web resources with proxy
on Crosswalk, but android system MediaPlayer can't set a proxy with a standard API.
The ExoMediaPlayer is playing videos and music is a popular activity on Android devices,
and it can be configured with proxy.
https://developer.android.com/guide/topics/media/exoplayer.html

BUG=XWALK-6770

media/base/android/java/src/org/chromium/media/MediaPlayerBridge.java

@@ -35,12 +35,18 @@
 */
 @JNINamespace("media")
 public class MediaPlayerBridge {
-
+    /**
+    * Give the host application a chance to take over MeidaPlayer.
+    */
     public static class ResourceLoadingFilter {
         public boolean shouldOverrideResourceLoading(
                 MediaPlayer mediaPlayer, Context context, Uri uri) {
             return false;
         }
+
+        public MediaPlayer getExternalMediaPlayer() {
+            return null;
+        }
     }
 
     private static ResourceLoadingFilter sResourceLoadFilter = null;
@@ -80,7 +86,8 @@ protected void destroy() {
 
     protected MediaPlayer getLocalPlayer() {
         if (mPlayer == null) {
-            mPlayer = new MediaPlayer();
+            mPlayer = sResourceLoadFilter.getExternalMediaPlayer();
+            if (mPlayer == null) mPlayer = new MediaPlayer();
         }
         return mPlayer;
     }
@@ -202,9 +209,9 @@ protected boolean setDataSource(
             headersMap.put("allow-cross-domain-redirect", "false");
         }
         try {
-            if (sResourceLoadFilter != null &&
-                    sResourceLoadFilter.shouldOverrideResourceLoading(
-                            getLocalPlayer(), context, uri)) {
+            if (sResourceLoadFilter != null
+                    && sResourceLoadFilter.shouldOverrideResourceLoading(
+                               getLocalPlayer(), context, uri)) {
                 return true;
             }
             getLocalPlayer().setDataSource(context, uri, headersMap);